look at the feat and the Alternative Favored Enemy as well ...:
You are the holy bane of creatures of a particular type.
Prerequisites: Favored enemy class ability.
Benefit: Choose one of your favored enemies. You can sense the
presence of creatures of this type within 60 feet, as well as pinpoint
their exact location (distance and direction) relative to you. Normal
barriers and obstructions do not block this supernatural ability,
allowing you to sense the presence and location of creatures
behind doors or walls, for example. This feat does not allow you to
see an invisible or hidden creature (although you can still discern
In addition to sensing the presence of your favored enemy,
you deal +1d6 points of damage on weapon attack rolls made
against evil representatives of the favored enemy creature type.
Special: You can choose this feat multiple times. Its effects do
not stack. Each time you select this feat, it applies to a different
Rangers learn to track and hunt creatures of a certain type
when choosing a favored enemy. Some rangers concern
themselves less with what a creature is, and more with what
it does. These rangers learn to identify signs of the arcane
and make it their business to combat those who use arcane
magic in opposition to their goals.
Special Requirement: Knowledge (arcana) 1 rank.
Replaces: At 1st level, you do not select a favored enemy
from Table 3-14 in the Flayer's Handbook. At 5th, 10th, 15th,
and 20th level, you can choose favored enemies as normal.
Benefit: At 1st level, you gain favored enemy (arcanists).
This feature works just like the favored enemy ability (PH
47). The bonuses granted apply to any character capable of
casting arcane spells or using invocations
(but not other
it also workson a lot of PrCs ... example: